1. Field of the Invention
The present invention relates to a motor drive circuit for a DC brushless motor; more particularly, the present invention relates to a CMOS motor drive circuit with high torque, good noise immunity and high efficiency,
2. Description of Related Art
The conventional DC brushless motor makes use of two stator coils for generating a rotating magnetic field to rotate the rotor of a fan. FIG. 9 shows a motor drive circuit for such a two-phase DC brushless motor. The motor drive circuit comprises a Hall effect element (Hall IC) (70) for detecting the polarity of a rotor formed from a permanent magnet in the motor, a first transistor (71) connected to a first magnetic coil (L1), and a second transistor (72) connected to a second magnetic coil (L2). In use of the motor drive circuit of FIG. 9, when the motor is powered on, the first transistor (71) is turned on and current is supplied to the first magnetic coil (L1) thereby generating a magnetic field to rotate the rotor. Then, the Hall IC (70) detects the change of magnetic polarity caused by the rotation of the rotor thereby generating a high level signal to turn off the first transistor (71) and turn on the second transistor (72) to supply current to the second magnetic coil (L2). The magnetic coil (L2) generates a magnetic field to further rotate the rotor. Consequently, the first transistor (71) and the second transistor (72) are alternatively turned on and turned off to supply current to the first magnetic coil (L1) and second magnetic coil (L2) respectively thereby continuously rotating the rotor.
The above conventional two-phase DC brushless motor suffers a disadvantage in having two stator coils, which results in higher manufacturing cost and larger motor dimensions. Therefore, there is a demand for having a single-phase DC brushless motor with a single coil. However, the above conventional motor drive circuit can not be used to drive such a single-phase motor. To overcome this problem, the U.S. Pat. No. 5,289,089 by Aoki discloses a motor drive circuit capable of driving both of the single-phase and two-phase motors. As shown in FIG. 10, this motor drive circuit comprises a Hall effect element (Hall sensor) (1), an amplifier (2) which receives output signals of the Hall sensor (1), and a drive pulse generating circuit (3) which receives the amplified signals of the amplifier (2), wherein the outputs of the drive pulse generating circuit (3) are connected to first and second drive circuits (11,12) which differ in phase. The first drive circuit (11) is constituted by transistors Q1 to Q3 and Q8 while the second drive circuit (12) is constituted by transistors Q4 to Q7. Further, switching diodes D1 and D2 are provided in parallel with the transistors Q3 and Q7 respectively. The emitters of the transistors Q3 and Q7 are connected to external terminals 6 and 7 respectively for further connection to a coil (10). In operation, the drive pulse generating circuit receives the amplified signal from the amplifier (2), generates first and second pulses having a duty of about 50% of which phases differ by about 180 degrees from each other and outputs the same. The first and second drive circuits respectively receive the first and second pulses and serve alternatively as a current flow out side and as current sink side with respect to each other to drive the motor. In addition to driving a single-phase motor as shown in FIG. 9, this motor drive circuit can be used to drive a two-phase motor by connecting two first terminals of the motor's two coils to external terminals 6 and 7 respectively and connecting two second terminals thereof to a power line Vcc.
Although Aoki's patent can be used to drive either a single-phase or a two-phase motor and thus overcomes the disadvantages of the conventional motor drive circuit for a two-phase motor, however, some unsatisfactory points still exist:
1. Restriction in use of the Hall element: It is well known that a Hall element can be either a Hall sensor or a Hall IC wherein the two differ in that the Hall IC further provides a signal amplifying capability so that the output signals of the Hall IC can be applied to circuitry without amplification. In the Aoki Patent, the amplifier 2 is provided in the motor drive circuit to amplify the output of the Hall element which is known to be a Hall sensor. Therefore, the Hall element of the Aoki Patent can not be a Hall IC.
2. Redundancy in circuit architecture: The diodes D1 and D2 connected in parallel with the transistors Q3 and Q7 respectively are provided to avoid generating switching noise resulting from the counter electromotive force on the motor coil (10). However, if the motor drive circuit is implemented in an IC, the two diodes (D1,D2) have to be built-in and require a significant area, resulting in a complicated manufacturing process and high manufacturing cost.
3. Short usage life: The major reason that makes a motor drive circuit over-heat and thus shortens its life is that the transistors therein are alternatively turned on and off. It is known that a delay may occur when an electronic element makes a transition. Therefore, if a delay occurs when a first transistor transits from on to off, a corresponding second transistor turns on before the first transistor is entirely turned off, which results in short-circuit and over-heating problems thereby shortening the life of the motor drive circuit.
4. Large power dissipation: Aoki's single-phase motor drive circuit includes bipolar devices in an IC form. Since the characteristics of PNP transistors are not as good as NPN transistors in most bipolar processes, Aoki's patent employs two NPN transistors (Q3,Q7) as pull up devices. However, there is a voltage drop V.sub.BE.backslash.Q3 +V.sub.CE.backslash.Q2 approximately equal to 1.0 volt generated between the power source Vcc and the external terminal 6. Similarly, a voltage drop V.sub.BE.backslash.Q7 +V.sub.CE.backslash.Q6 approximately equal to 1.0 volt is also generated between the power source Vcc and the external terminal 7. These voltage drops will result in larger power dissipation and generate more heat.
5. Easy to be disturbed by noise: The output signals of the Hall sensor (1) in Aoki's patent are directly sent to the drive pulse generating circuit (3) after amplifification. There is no error detecting mechanism provided. Therefore, the motor drive circuit is likely to make mistakes if disturbed by noise.
Based on the unsatisfactory points described above, there is a need to improve the motor drive circuit